BH formation efficiency of the clusters, defined as the mass of the most massive BH divided by the total BH mass of the cluster, as a function of the ratio between the rms velocity at the time of the core collapse and the speed of light (c) employed in the simulations. Different colors are used to denote varying external potentials, η_g. The vertical lines mark velocity ratios assuming the real value of the speed of light for clusters with an rms velocity of 1000 km/s and 3000 km/s.